1. Field of the Invention
The present invention relates to a polarization mode dispersion compensator for compensating waveform distortion of an optical signal due to polarization mode dispersion received in an optical transmission line and a compensation method for polarization mode dispersion.
2. Description of the Related Art
With the advance of construction of optical fiber communication networks, still higher speed transmission is required. For the development of high speed transmission various kinds of studies are being performed; however, as one of the causes hindering the development of high speed transmission, the polarization mode dispersion in an optical transmission line is at issue. PMD (Polarization Mode dispersion) is a phenomenon in which transmission delays in 2 polarization eigen modes to be the principal axes are different, in a case where an optical fiber or an optical device to be used in a transmission line has polarization depenence. The difference in transmission delays between the 2, xcex94 T, expresses the amount of polarization mode dispersion. The waveform of a received optical signal is observed as the sum of 2 waveforms, so that the waveform of the received optical signal is distorted. Thereby, the eye-pattern of the received waveform is also largely distorted and can cause a big problem such as impossibility of transmission or degradation of transmission characteristics, degradation of receiving sensitivity, etc. The principal axes of polarization or the amount of polarization dispersion of a transmission line is varied by the pressure or vibration given to an optical fiber, the environmental temperature, so that the quantity of degradation due to polarization mode dispersion also varies every moment. several compensation methods as countermeasures against polarization mode dispersion have been proposed. For example, in the paper, T. Takahashi, etc., xe2x80x9cAutomatic Compensation technique for timewise fluctuating polarization mode dispersion in in-line amplifier systems,xe2x80x9d IEE Electronics Letters, Vol. 30, No. 4, February, 1994, pp 348-349, an optical compensation method for polarization mode dispersion is proposed. In this method, an optical signal degraded by polarization mode dispersion is received by an optical receiver through a compensation circuit for polarization mode dispersion. The compensation circuit for polarization mode dispersion is provided with a polarization controlling optical circuit and a polarization maintaining fiber for the polarization mode dispersion compensation, and the polarization state of an optical signal is controlled by the polarization controlling optical circuit and the incident state onto the polarization mode maintaining fiber for the polarization mode dispersion for the compensation is varied; thereby, the waveform distortion of the optical signal received during the transmission by polarization mode dispersion is canceled.
The control of the compensation circuit for polarization mode dispersion is executed as shown below. At first, a part of the output light from the compensation circuit for polarization mode dispersion is divided by an optical coupler and converted into an electrical signal by an optoelectric converter (O/E). After that, the magnitude of a frequency component of Rb/2 in the signal spectrum is observed by a frequency detector (center frequency: Rb/2; Rb: the bit rate of the transmitted signal). The state of the polarization controlling optical circuit is controlled to make the output signal of the frequency detector maximum by a maximum control circuit. The magnitude of the frequency component of Rb/2 in the signal spectrum becomes smaller as the eye-opening is made smaller by the degradation of the waveform due to polarization mode dispersion. Therefore, it is always possible to perform compensation for making the quantity of degradation of a received optical signal due to polarization mode dispersion minimum by constituting a feedback system as mentioned in the above.
In the conventional compensation system for polarization mode dispersion, there are some problems in applying it to an actual case. For example, for an O/E converter or a frequency detector, optoelectric circuits are needed which are able to process high speed signals of the same order as the transmitted signals, which causes demerits of the system from the point of production costs or the constitution of the system. In particular, the O/E converter requires a band width in the order of the transmission bit rate ( greater than G bit), and input sensitivity is lowered with the increase in bit rate. Therefore, it is needed to raise input optical power or dividing ratio of an optical coupler, which causes problems such as high cost or loss increase. Further, these component parts are selected according to the bit rate of an optical signal to be transmitted, so that a compensation circuit or component parts are not possible to use in common for the optical signals of different bit rates.
The intensity of the frequency component of Rb/2 to be used as control quantity is varied by the waveform degradation due to polarization mode dispersion, etc. or the mark ratio of a bit pattern to be transmitted, which causes a problem that the intensity of the frequency component of Rb/2 is easily affected by external disturbances. Further, when the amount of polarization mode dispersion of a transmission line approaches a bit width, the distortion of a waveform becomes too large and the intensity of the frequency component of Rb/2 becomes not to correspond to the amount of polarization mode dispersion. Thereby, the feedback control becomes not to work properly.
Therefore, the object of the present invention is to offer a polarization mode dispersion compensator and a compensation method having simple constitution and being strong against external disturbances.
The above-mentioned object can be achieved by a polarization mode dispersion compensator being provided with a compensation circuit for polarization mode dispersion for compensating waveform distortion of an optical signal due to polarization mode dispersion received in an optical transmission line, a degree of polarization measuring circuit for measuring the degree of polarization of the optical signal, and a control circuit for controlling the compensation circuit for polarization mode dispersion based on the obtained degree of polarization. In this place, the word, degree of polarization is used as a concept including some physical quantity expressing the extent of polarization besides the degree of polarization. In other words, the deal of degree of polarization in the circuit constitution is not necessarily the degree of polarization of an optical signal, the above-mentioned physical quantity can be dealt as the degree of polarization as far as it can be converted into the degree of polarization by calculation, etc. Its concrete example will be described later.
To be concrete, for example, a compensation circuit for polarization mode dispersion is constituted with a polarization controller which varies the polarization state of an optical signal and a polarization mode dispersion element connected to the polarization controller. As a polarization mode dispersion element, a polarization maintaining fiber can be used. The degree of polarization measuring circuit can be constituted with a polarization state analyzer for detecting the polarization state of an optical signal and a degree of polarization calculating circuit for calculating the degree of polarization using the polarization information obtained by the polarization state analyzer. There is another constitution, in which the degree of polarization measuring circuit can be constituted with a polarization controller which varies the polarization state of an optical signal, a polarization dependence element connected to the polarization controller and a signal processing circuit for calculating the degree of polarization using the output of the polarization dependence element. It is recommendable to provide an optical band pass filter including the optical signal to be the subject of compensation in its pass band in the preceding stage of the compensation circuit for polarization mode dispersion.
In the compensation method for polarization mode dispersion according to the present invention, at first, the degree of polarization of an optical signal to be transmitted in an optical transmission line is found, and the waveform distortion of the optical signal due to polarization mode dispersion received in the optical transmission line is compensated using the obtained degree of polarization. In this case, the optical signal is so compensated as to make the degree of polarization of the optical signal a desired value. Generally, an optical signal is so compensated that the above-mentioned desired value of the degree of polarization is the maximum value in the optical transmission line. The degree of polarization is naturally restricted by the characteristics of the optical transmission line. Therefore, the maximum value of the degree of polarization mentioned in the above means the maximum value which can be realized with the characteristics of the optical transmission line. Further, it is also possible to find the degree of polarization using at least one of the modulation information such as amplitude, phase, frequency, polarization or a code, which is given to the optical signal beforehand.
In the optical fiber communication system according to the present invention, a plurality of optical transmission devices are connected to each other with optical fibers, and at least one of these devices is provided with a polarization mode dispersion compensator for compensating the waveform distortion of an optical signal due to polarization mode dispersion using the degree of polarization of the optical signal. In the same way, the present invention can be applied to a wavelength division multiplexing transmission system. The optical transmission device used in this place is constituted with an optical receiver connected to the optical transmission line and a polarization mode dispersion compensator for compensating the waveform distortion of an optical signal using the degree of polarization of the optical signal provided in the preceding stage of the optical receiver. Thereby, it is made possible to obtain a polarization mode dispersion compensator and a compensation method for polarization mode dispersion having a simple constitution and being strong against external disturbances.